Finite width graphene strips, which are referred as graphene
nanoribbons, have been also studied attentively
[1]}, [2]}, [3]}, [4]}, [5]}. It has been shown that
GNRs of various widths can be obtained from graphene monolayers
using patterning techniques [4]}. The transport properties
of the perfect GNRs are expected to depend strongly on whether they
have zigzag or armchair edges [1]}. In the framework of the
nearest neighbor tight binding (NNTB) model, the GNRs with zigzag
edges are always metallic while the armchair structures are either
semiconducting or metallic depending on their width. In the zigzag
GNRs, the bands are partially flat around the Fermi energy [\(E = 0\) 
eV], which means that the group velocity of conduction electrons is
close to zero. Their transport properties are dominated by edge
states [3]}. In the GNRs with armchair edges, the bands
exhibit a finite energy gap in the semiconducting structures or are
gapless in the metallic ones [1]}. However, ab initio
studies have demonstrated that there are no truly metallic armchair
GNRs (see in Ref. [2]} and reference therein). Even for the
structures predicted to be metallic by the NNTB model, a small
energy gap opens, thus modifying their behavior from metallic to
semiconducting. In general, the group velocity of conduction
electrons in armchair GNRs is high, e. g., it is constant and equal
to about \(10^6\)  m/s [11]} in those metallic structures. The
transport properties of ideal zigzag and armchair GNRs are thus very
different. In the current work, we pay our attention only to the
ribbons with armchair edges.
